The present invention relates to a process for the production of five-membered or six-membered cyclic ethers, in particular of anhydropolyols, by acid-catalysed cyclodehydration of polyols that contain at least two hydroxyl groups with a spacing enabling ring formation--that is to say, preferably with a spacing of 4 or 5 C atoms. The invention is directed in particular towards the production of anhydrotetritols, anhydropentitols and, particularly preferred, anhydrohexitols, from tetritols, pentitols and hexitols, respectively.
The acid-catalysed cyclodehydration of polyhydric alcohols accompanied by the formation of, in particular, 5-membered cyclic ethers or hydroxyethers, designated hereinafter as anhydropolyols, has been known for a long time. The cyclodehydration of a sugar alcohol from the family of the hexitols results in a complex product mixture of anhydrohexitols and dianhydrohexitols as well as undesirable by-products arising by virtue of the production process, among them polymers; see K. Bock et al. in Acta Chemica Scandinavica B 35 (1981) 441-449 and G. Fleche et al. in Starch/Starke 38 (1986) No. 1, 26-30. Increasing interest is being shown in anhydropolyols that can be produced from renewable raw materials such as sugars, for instance 2,5-sorbitan and isosorbide from D-glucose or sucrose via sorbitol, in connection with the production of polyester resins, epoxy resins and surfactants.
In the article by G. Fleche cited above, the influence of the water content, the type of acid and the acid concentration on the composition of the product and the polymer content in connection with the acid-catalyzed cyclodehydration of sorbitol obtained by hydrolysis of starch and hydrogenation of the D-glucose arising in the process is dealt with. This article recommends that dehydration be carried out, as far as possible, in the absence of water. Mineral acids, organic cation-exchangers or Lewis acids are employed as catalysts. The use of mineral acids or Lewis acid as catalysts necessitates neutralization and elaborate separation of the salt from the reaction mixture and disposal of the salt. A disadvantage of all forms of implementation is the usually high polymer content in the reaction mixture. The polymer content is particularly disadvantageous when the reaction mixture is to be supplied directly--that is to say, without further elaborate purifying stages--to a stage for further utilization after removal by distillation of the isosorbide which is formed.
According to DE-OS 31 11 092 cyclodehydration can also be carried out by means of gaseous hydrogen halide such as HCl as catalyst and, optionally in addition, of an organic carboxylic acid as co-catalyst. Disadvantages of this process include the very large quantity of catalyst required, as well as the small proportion of monoanydro products produced. According to WO 89/00162 the cyclodehydration of hexitols can also be carried out at moderate temperature in liquid hydrogen fluoride in the presence of a carboxylic acid, but this process is very elaborate on account of the great dangers of HF to both people and material.
Instead of using acidic catalysts, the cyclodehydration of polyols such as glucitol can also be catalyzed by means of bimetallic catalysts such as Cu--Pt, Cu--Au, Cu--Pd and Cu--Ru in the presence of hydrogen; see C. Montassier et al., Applied Catalysis A: General 121 (1995), 231-244. However, in the course of this conversion, with increasing conversion of glucitol (=sorbitol) into monoanhydroglucitols and 1,4:3,6-dianhydroglucitol a decline in dehydration selectivity occurs. Although the selectivity can be increased again by addition of NaCl, the activity of the catalyst falls off considerably in the process.
According to EP-B 0 380 402 it is possible for a maximum of 71% anhydro compounds (49% isosorbide and 22% isomeric monoanhydroglucitols) to be obtained from glucitol. Disadvantages include the high loss (29%) of parent compound, the long reaction times required, and the high catalyst demand or, as the case may be, the necessity for catalyst regeneration. Unless long reaction-times are taken into consideration, the conversion of sorbitol is incomplete, so that the cyclodehydration reaction mixture also contains, besides the monoanhydrohexitols and isosorbide, considerable amounts of sorbitol. Although isosorbide can easily be separated from the mixture by distillation, besides the monoanhydrohexitols the distillation sump remaining contains sorbitol, this latter having a detrimental influence on consequent conversions or necessitating elaborate purification.
Therefore, an object of the present invention is to carry out a process for the cyclodehydration of polyols, in particular of sugar alcohols, that results in substantially quantitative conversion and that leads to cyclodehydration products containing less than 1 wt % polymers.